1. Field of the Invention
The present invention relates to a method for manufacturing a heterojunction bipolar transistor (HBT), and more particularly, to a method for manufacturing a BiCMOS by combining a process of manufacturing an HBT and a process of manufacturing a complementary metal oxide semiconductor (CMOS) transistor.
2. Description of the Related Art
Laborious research into high-frequency operation transistors has progressed along with developments in high-speed communications technologies. In recent years, a SiGe HBT has been applied to high-frequency operation transistors. The SiGe HBT is different from a typical bipolar transistor in that a SiGe epitaxial layer forms a base. When SiGe, which has an energy bandgap that is smaller than that of Si, is used as the base of the HBT, current gain and operating speed can be improved. Even if a high-concentration of impurity ions are implanted into the base, current gain is not lowered. Also, base resistance is reduced so as to decrease figure of noise, and operating voltage is reduced to enable low power dissipation. Further, transition frequency fT and maximum frequency fmax can be increased by controlling the content and distribution of Ge in the SiGe. For these reasons, in communications devices or phase-locked loops (PLLs), a SiGe HBT is widely used as a high-frequency operation device in which fT and fmax are 50 GHz or higher.
An HBT is typically combined with a CMOS transistor to constitute a BiCMOS device. A well-known BiCMOS is formed by combining a CMOS transistor and a bipolar transistor on a silicon substrate. Advanced BiCMOS technologies utilize a SiGe HBT in place of a bipolar transistor. SiGe bases are used for high-performance HBTs which process analog signals, while CMOS transistors are used to process digital signals and store data.
A SiGe HBT exhibits the same performance as Group III-V semiconductor compositions and can be fabricated economically using conventional silicon fabrication processes. Also, “system-on-chip” is enabled using a silicon semiconductor process, thus increasing applicability.
However, as fabrication of a SiGe HBT employs a conventional silicon semiconductor process, misalignment may occur during a photolithographic process, thus degrading performance of a transistor. For example, the doping profile or the area of an emitter-base junction or a base-collector junction may vary performance.
Accordingly, self-alignment is required to maintain minimal variation in the performance of a transistor caused by junctions. Self-alignment, which is typically used to form source/drains of CMOS transistors, can be implemented using spacers.
In particular, conventionally, a double poly structure or outer spacers are used to facilitate self-alignment. However, in this case, the area of a transistor may be increased, thus lowering integration density, or dummy spacers may be formed, thus leading to more defects in a fabrication process. To solve the foregoing problems, a method of forming an emitter-base junction using high-pressure oxidation (HiPOX), which can oxidize polysilicon at a low process temperature, was proposed. However, the HiPOX process generates particles, thus lowering yield.